A Passive Variable Switching Frequency SPWM Concept and Analysis for DCAC Converter

Variable switching frequency sinusoidal pulsewidth modulation (VSFSPWM) method does well in many aspects such as lower harmonic peak and losses compared with constant switching frequency sinusoidal pulsewidth modulation (CSFSPWM) for DCAC converters. This article presents a passive VSFSPWM (PVSFSPWM) method for spectrum improvement, filter inductor reduction, and losses reduction, obtaining similar output current quality as that of the CSFSPWM method. Because the PVSFSPWM method does not need any feedback information compared with the model-predictive-based method, it is an open-loop method and is called the passive method. In order to obtain the law of harmonic distribution, the mathematical expression of harmonic spectrum distribution is derived. The required filter inductor under the same current ripple and the losses between CSFSPWM and PVSFSPWM methods have been compared. Simulation and experimental results show that the PVSFSPWM method achieves excellent performance in EMI suppression, inductance, and losses reduction without impairing the output quality.